1. Field of the Invention
This invention relates to a method of growing a mixed compound semiconductor by vapor phase epitaxy and an apparatus used therefor, and more particularly to the method of growing a group II-VI mixed compound semiconductor layer of Hg.sub.1-x Cd.sub.x Te and an apparatus used therefore.
The compound semiconductor Hg.sub.1-x Cd.sub.x Te has a characteristic of a small energy bandgap and is known as a detector material for infrared rays. The mixing ratio x included in the above expression Hg.sub.1-x Cd.sub.x Te is defined as a ratio of binary compound semiconductor CdTe comprised in ternary compound semiconductor HgCdTe and plays an important role to determine the most sensitive wavelength of infrared rays in detection. The present invention relates to the method and apparatus for growing a compound semiconductor layer of Hg.sub.1-x Cd.sub.x Te having a uniform x-value throughout the entire grown crystal layer on a substrate.
2. Description of the Related Art
A general concept of growing a mixed compound semiconductor layer of Hg.sub.1-x Cd.sub.x Te using an apparatus comprising a reactor chamber of a vertical type is first explained using FIG. 1. FIG. 1 shows a schematic cross sectional view of the apparatus, wherein a substrate 3 of such as gallium arsenide (GaAs) is disposed on a substrate stage 2 which is rotated around the axis of a center support 10 during the growth. A mixed source gas for growth is supplied into the reactor chamber 1 through a nozzle 4 and the source gas is spouted out vertically onto the substrate 3. In FIG. 1, only a single nozzle is illustrated, however, a plurality of nozzle can be substituted therfor. The reactor chamber 1 is arranged on a fixed flange 6 with which an outlet pipe 7 is provided for exhaust. A RF coil 5 is arrange outside the reactor chamber 1 for heating the substrate stage 2 which is made of graphite.
A plurality of gas sources are provided which are shown in FIG. 2. A mercury (Hg) bubbler 30 supplies mercury vapor contained in a bubbling hydrogen gas. The mercury bubbler 30 comprises a valve 22, a mass flow controller 23, and a pressure gauge 28. A hydrogen gas is supplied to two mass flow controllers 23 and a partial pressure of mercury contained in the mixed hydrogen source gas is precisely controlled. The similar bubblers 32 and 34 for tellerium and cadmium are respectively also provided, but the details are omitted in FIG. 2 for simplicity. A di-isopropyltelluride (abbreviated hereinafter as DIP-Te) bubbler 32 supplies DIP-Te vapor contained in a bubbling hydrogen gas. A dimethyleadmium (abbreviated hereinafter as DM-Cd) bubbler 34 supplies DM-Cd vapor contained in a bubbling hydrogen gas.
During the growth, the above three source gases are mixed and introduced into the reactor chamber 1 through the gas nozzle 4, and the mixed gas is spouted out onto the substrate 3. The rotating substrate stage 2 together with the substrate 3 is heated by the RF coil 5. DIP-Te and DM-Cd source gases are decomposed into element gases, and the decomposed elements and Hg of the Hg source gas react with each other in the reactor chamber 1 forming mixed compound semiconductor Hg.sub.1-x Cd.sub.x Te, which deposits on the substrate 3 epitaxially on the substrate. The method falls under the category called MOCVD (Metal Organic Chemical Vapor Deposition).
The above method includes the problem that, when source gases are heated in the reaction chamber 1, binary compound semiconductor CdTe is formed much easier than the formation of binary compound semiconductor HgTe. Most of decomposed Cd molecules are consumed on the substrate surface area directly under the nozzle 4, since formation energy of CdTe is smaller than that of HgTe. This results in forming Hg.sub.1-x Cd.sub.x Te compound layer which has a higher x-value on the central portion of the substrate 3 and a lower x-value on the peripheral substrate area thereof. Therefore, the x-value of the grown ternary compound semiconductor Hg.sub.1-x Cd.sub.x Te is not uniform on the substrate surface.
This is schematically shown in FIG. 3. The abscissa shows a position on the substrate 3 which has a diameter 2r, and the ordinate shows an x-value of the grown compound semiconductor Hg.sub.1-x Cd.sub.x Te layer. FIG. 3 shows that the grown ternary semiconducotr Hg.sub.1-x Cd.sub.x Te has a composition such that the x-value thereof has a peak value at the center of the substrate 3 and gradually decreases toward the periphery thereof.
A plurality of nozzles are used in order to improve a uniformity in x-values of the grown semiconductor. An exemplary result is shown in FIG. 4, in which four nozzles 4-1 to 4-4 are utilized. The result shows a remarkable improvement compared with that shown in FIG. 3, however, fluctuation in x-values is not improved to a satisfactory level.
In order to improve uniformity of the grown semiconductor, the following patent application have been published. Japanese Unexamined Patent Publication SHO 62-297296 published Dec. 24, 1987 discloses an MOCVD apparatus, wherein a substrate is disposed on a susceptor and rotates around the center axis thereof, the substrate surface is arranged to be parallel to the gas flow direction, and further the susceptor can move back and forth in the transversal direction to the gas flow. Japanese Unexamined Patent Publication HEI 1-140712 published Jun. 1, 1989 (the same patent was filed to the U.S. Patent and Trademark Office and allowed as U.S. Pat. No. 4,980,204) discloses a CVD apparatus similar as that described in FIG. 4, wherein each gas flow through a plurality of nozzles is individually and precisely controlled. Further, Japanese Unexamined Patent Publication HEI 3-271195 published Dec. 3, 1991 discloses a mechanism for moving a substrate stage in a CVD apparatus, in which a substrate disposed on a substrate stage rotates around the center axis of the substrate and the rotating substrate stage further receives around the center axis of the CVD apparatus, whereby the substrate stage moves back and forth inwardly and outwardly in the radial direction.